With the development of mobile electronic devices such as mobile phones or laptop computers, there is a dramatic increase in the demand for rechargeable secondary batteries as a source of energy for the mobile electronic devices. Recently, the use of secondary batteries as a source of power for hybrid electric vehicle (HEV) and electric vehicle (EV) is realized in practice. Accordingly, many studies are being made on secondary batteries that meet various demands, and in particular, the demand for lithium secondary batteries having high energy density and high discharging voltage and output is on the increase.
Lithium ion secondary batteries include carbon-based materials such as graphite, lithium metal, metal such as tin or silicone or their oxide, alloys including them as a negative electrode active material. Among them, lithium metal is more prone to ionization and has a body-centered cubic crystal structure and an atomic radius of 0.76 Å. Furthermore, lithium metal has small atomic mass (6.941) and low density (0.534 g/cc) as well as very low standard electrode potential (−3.04 VSHE), so its specific capacity is very high as much as about 3860 mAh/g. However, lithium metal has a generally low melting temperature of 180.54° C., and has a safety problem, for example, an internal short of batteries caused by dendrite growth. To solve this problem, studies have been made to stabilize the surface of lithium metal by coating the surface with an electrochemically stable material, for example, a polymer or inorganic film. U.S. Pat. No. 4,503,088 forms a protective coating by applying an epoxy resin solution to a lithium metal negative electrode, but there is a high likelihood that the solvent in the solution may directly contact the lithium metal, resulting in by-product formation, and bubbles are generated at the interface. Besides, explosion reaction that may occur due to the exposure to moisture and difficulties in electrode manufacturing process are at issue.